early 15c., "bishop having oversight of other bishops," from Late Latin metropolitanus, from Greek metropolis "mother city" (from which others have been colonized), also "capital city," from meter "mother" (see mother (n.1)) + polis "city" (see polis).
In Greek, "parent state of a colony;" later, "see of a metropolitan bishop." In the West, the position now roughly corresponds to archbishop, but in the Greek church it ranks above it.
1540s, "belonging to an ecclesiastical metropolis," from Late Latin metropolitanus, from Greek metropolites "resident of a city," from metropolis (see metropolitan (n.)). Meaning "belonging to a chief or capital city" is from 1550s. In reference to underground city railways, it is attested from 1867.